1. Field of the Invention
The present invention relates generally to chemical milling of metallic materials. More particularly the invention concerns a unique method and apparatus for automatically measuring, scribing, chemically milling and inspecting sheet metal workpieces including workpieces having a compound curved surface.
2. Discussion of the Prior Art
Chemical milling may be defined as a process of etching the surfaces to be milled by chemical attack. The techniques for chemical milling of metallic workpieces are well known and have proven particularly useful in the past for applications wherein it is desired to remove specific amounts of material in predefined areas of aluminum, magnesium, titanium or steel sheet material after the sheet has been either rolled or stretch formed. As a practical matter, it is not feasible to mechanically mill large sections of sheet material, and particularly sheet material having a compound curved surface, due to equipment limitations and great expense. However, in many applications, including aerospace applications, where part weight and wall thickness tolerances are critical, precision milling of large sheet metal components is frequently required. Chemical milling has proven quite valuable and is widely used in such applications.
The standard approach followed in the past in chemical milling sheet workpieces to a uniform wall thickness was to first measure the wall thickness of the part at a multiplicity of points. The wall thickness data thus obtained was then used to draw contour lines on the surface of the part which represented regions of greater and lesser wall thickness.
Initially, the thickness gaging step was accomplished through the use of an ultrasonic transducer which, when coupled by a wet film to the surface of a metal plate, could measure thickness by its relation to the time between energy pulse echos from the two surfaces of the material being scanned. More recently the non-destructive testing industry has developed a system whereby a focused energy beam can be made to travel to and echo from a metal part within a moving column of water which impinges on the surface of the part and functions as the beam carrier medium. This advantageously enables the thickness measurements to be taken without the probe coming into physical engagement with the surface of the part. Exemplary of such a thickness measurement device is a unit manufactured and sold by NDT Instruments of Huntington Beach, Calif.
After the contour lines were drawn on the surface of the part, the next step in the prior art procedures was to cover the surface of the part with a thin film of vinly plastic, gelatin, rubber base material, or other etch-proof film, or maskant. This was done by spraying, painting, dipping or otherwise applying the maskant to the surface of the part. Due to the substantial transparency of the maskant, the contour lines drawn on the part surface remained visible. Next, using a sharp knife or razor blade, a portion of the maskant was cut away by hand as, for example, along the contour lines of an area of greater wall thickness. The part was then immersed into the etching bath which comprised acid, a suitable caustic, or other chemical attacking means. Since the maskant protected all the surface save the unprotected area, only this area would be attacked by the chemical and would be milled away. The amount of material removed was, of course, dependent on several factors, such as temperature, time, chemical concentration and the type of starting material. However, those skilled in the art were able to precisely and uniformly control the amount of material which was removed. Successive steps of cutting away the maskant from other portions of the part, reimmersing of the part into the etching bath and continued gaging of the etched areas permitted precise milling of the surface of the part to a desired uniform wall thickness. A typical prior art technique for chemical milling using a polyvinyl maskant is described in U.S. Pat. No. 2,739,047 issued to Manuel C. Sanz.
After the part was milled to the desired wall thickness, it was then frequently necessary to repeat the chemical milling steps to provide engineering features such as ribs, embossments, lands and the like at specified locations on the part. To mill these engineering features it was, of course, necessary to re-mask the part, selectively cut away portions of the maskant and successively immerse the part in the etching solution to form the required engineering features.
Particularly with large parts, the time required to gage and mark the surface areas to be etched was highly labor intensive, often involving many man hours. Similarly, the repeated spraying, painting or otherwise covering the part surface with maskant to accomplish the wall thickness milling and the milling of the engineering features was time consuming, costly and frequently troublesome and hazardous. Finally, the cutting of the maskant by hand followed by the successive etching, rinsing and recutting steps was tedious, time consuming and most cost ineffective. It is these and other drawbacks of the prior art processes which have been uniquely overcome by the novel method and apparatus of the present invention.
As will become apparent from the discussion which follows, in accordance with the method of the present invention, the as received part is initially covered with maskant and the entire wall thickness gaging is accomplished automatically and in a highly novel manner by a rectilinear type robot, or similar robotic device. During the gaging step, all the wall thickness data taken by the robot is entered into a host computer for manipulation and later recall. Next the data defining the specific engineering features desired on the particular part is entered into the computer. The gaging sensor carried by the robot is then replaced with a suitable cutting device, such as a low power laser. The rectilinear robot is then drivably interconnected with the computer through a robot controller and the maskant is automatically cut along selected lines in accordance with the wall thickness and engineering data previously entered into the computer. The part is then milled in the exposed areas to simultaneously achieve both the desired wall thickness and engineering features. After all the required maskant cutting and milling steps have been completed, the gaging sensor is once again mounted on the robot and the part is automatically inspected and the inspection data is entered into the computer for verification of compliance with proscribed specifications.